Supplementary Data on Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities
Net income	$ 30,068	$ 25,357	$ 20,452	$ 28,366	$ 34,287	$ 23,599	$ 25,995	$ 27,012	$ 31,826	$ 32,266	$ 13,103	$ 26,671	$ 104,243	$ 110,893	$ 103,866
Adjustments to reconcile net income to net cash provided by operating activities:
Realized investment gains													(10,917)	(22,245)	(27,298)
Increase in accrued investment income													(5,563)	(1,898)	(2,618)
Increase in accrued expenses													1,513	1,157	13,589
Depreciation and amortization													7,958	7,680	7,892
Increase in insurance liabilities													153,423	143,542	127,992
Increase in premium receivables													(3,638)	(4,018)	(5,638)
Increase in deferred policy acquisition costs													(12,662)	(14,659)	(13,989)
(Increase) decrease in reinsurance recoverable													1,570	(1,289)	872
Increase in income tax liabilities													9,745	7,099	29,752
Other													(23,739)	(20,326)	(31,572)
Total adjustments													117,690	95,043	98,982
Net cash provided by operating activities													$ 221,933	$ 205,936	$ 202,848